This invention relates to the deposition of thin films, and, more particulary, to sources for cluster beams.
The deposition of thin films upon substrates is an important manufacturing and research tool in a variety of fields. For example, microelectronic devices are prepared by depositing succssive film layers onto a substrate to obtain specific electronic properties of the composite. Photosensitive devices such as vidicons and solar cells are manufactured by depositing films of photosensitive materials onto substrates. Optical properties of lenses are improved by depositing films onto their surfaces. These examples are, of course, only illustrative of the thousands of applications of thin film deposition techniques.
In the highly controlled approach to thin-film deposition that is characteristic of applications where a high quality film is required, the film is built up by successive deposition of monolayers of the film, each layer being one atom thick. The mechanics of the deposition process can best be considered in atomistic terms. Generally, in such a process the surface of the substrate must be carefully cleaned, since minor contaminant masses or even contaminant atoms can significantly impede the deposition of the required highly perfect film. The material of the film is then deposited by one of many techniques developed for various applications, such as vapor deposition, electron beam evaporation, sputtering, or chemical vapor deposition, to name a few.
In another technique for depositing thin films, ionized clusters of atoms (or molecules) are formed in a cluster deposition apparatus. These clusters usually have on the order of about 1000-2000 atoms each. The clusters are ionized and then accelerated toward the substrate target by an electrical potential that imparts an energy to the cluster equal to the accelerating voltage times the ionization level of the cluster. Upon reaching the surface of the substrate, the clusters disintegrate at impact into atoms free to meve on the surface. Each atom fragment remaining after disintegration has an energy equal to the total energy of the cluster divided by the number of atoms in the cluster. Prior to disintegration the cluster therefore has a relatively high mass and energy, while each atom remaining after disintegration has a relatively low mass and energy. The energy of the atom deposited upon the surface gives it mobility on the surface, so that it can move to kinks or holes that might be present on the surface. The deposited atom comes to rest in the imperfections, thereby removing the imperfection and increasing the perfection and density of the film. Other approaches to using clusters have been developed, and thin film deposition using cluster beams is a promising commercial film manufacturing technique.
The cluster source, which produces the clusters, is a key component of a cluster beam deposition apparatus. The cluster source should produce a high mass flow of clusters of a selected size range, and exhibit a high cluster-forming efficiency. That is, the cluster beam should have a large fraction of the mass of the beam present as clusters rather than atoms, or the beneficial effect of using clusters may be lost. The cluster source should also provide a cluster beam with the clusters in the proper energetic state.
In one type of cluster source, the evaporative source, atoms are evaporated from a crucible within the source and fill the interior of the source. The source has an opening which functions as a nozzle through which a beam of clusters is emitted, and the nozzle produces a back pressure within the source so that the beam is emitted under pressure. The expansion of the beam through the nozzle has generally been thought to be the cause of the formation of clusters, as the reduced pressure in the expanded beam could cause homogeneous nucleation of clusters. A mixture of clusters and unclustered atoms is emitted from the source and progresses through the deposition apparatus to be deposited upon a target. The evaporative cluster source is distinguished from a carrier gas cluster source, wherein atoms are also evaporated but are mixed with a stream of a carrier gas to accelerate formation of clusters.
Existing evaporative cluster sources have suffered from the disadvantage of having a low cluster forming efficiency. That is, a high percentage of the atoms in the beam emitted from the source are emitted as unclustered atoms. After having been ionized, these atoms must be removed from the beam, or they will carry a high energy per atom to the target that defeats the purpose of using a cluster beam. A low percentage of atoms present as clusters also slows deposition of clusters, reducing the commercial efficiency of the deposition apparatus.
There therefore exists a need for a new type of evaporative cluster source having improved cluster forming efficiency. The present invention fulfills this need, and further provides related advantages.